Urea is today the main nitrogenous fertilizer used worldwide. Urea that is chemically pure has a nitrogen content of 46.6% N (expressed as N). Urea that is commonly available and used as fertilizer is usually very pure and typically has a nitrogen content of 46% N. Urea is therefore the fertilizer with the highest concentration of nitrogen, one of the reasons of its popularity.
Urea can be applied as such for straight nitrogenous fertilization, or in combinations with other elements, such as for example NS grades in which nitrogen and sulphur sources are combined, NP (respectively NK) grades in which nitrogen and phosphate (respectively potash) sources are combined, NPK combining the three major nutrients required by the crops, etc.
These different elements can be combined with urea as a physical blend of different products, or products being mixed/processed together into homogeneous granules by e.g. granulation, compaction, etc. Some examples of urea-based products :                NS products such as UAS which are mixtures of urea and ammonium sulphate, for example with a N content of 40% N,        NPK triple 19 expressed in N/P2O5/K2O, which are combinations of urea, DAP (di-ammonium phosphate) and muriate of potash (MOP),        etc.        
Urea and urea-based compounds are often blended with other fertilizers in order to adjust the formula and thus supply a balanced nutrition with the different elements required for the plant growth. The main advantages of blending are of course that from a limited amount of available products, it is possible to produce virtually an infinite amount of grades adjusted to the requirement, in a very flexible way.
However, blending of urea and of urea-based compounds is sometimes difficult or even impossible, due to products chemical incompatibility. Reference is made to the well documented and explicit “Guidance for the compatibility of fertilizer blending materials” published by EFMA in June 2006.
In particular, urea and urea-based compounds used as fertilizer, are well known to be not blendable with ammonium nitrate and ammonium nitrate based product (CAN, NPK, etc.), as well as with superphosphates (single superphosphate SSP, triple superphosphate TSP, etc.). Urea and urea-based compounds are also known to be difficult to blend with calcium nitrate.
These blending incompatibilities or limitations have different causes.
When blending urea-based products together with ammonium nitrate based compounds, the mixture will quickly become wet and absorbs moisture from the surrounding atmosphere, turning the free flowing granules into a wet mud. Even if moisture absorption from the surrounding atmosphere is prevented, the blend will turn wet from its own water content present from the beginning. The reason is that urea and ammonium nitrate form a double salt that is especially hygroscopic. As soon as urea and ammonium nitrate are in contact, this double salt is forming and starts turning liquid. Being more hygroscopic than the initial constituents, it will attract the moisture from the rest of the blend. The formed liquid phase will dissolve the products in contact, thus forming more UAN double salts and enhance the phenomenon that will propagate further.
The incompatibility effect with for example superphosphates and calcium nitrate is different. Many salts contain some water of crystallization, such as superphosphates and calcium nitrate. In presence of such salts, urea has the general tendency to form double salts, thus releasing the water of crystallization. Therefore mixtures of urea and urea-based compounds with SSP/TSP and with calcium nitrate will also have the tendency to turn muddy, independently from moisture pick up from the surrounding.
If these products are very dry, then they have the possibility to bind some of the water that would be released from the forming double salts with urea, making the blend still feasible. This is the reason why in the pre-mentioned blending guidance of EFMA, the remark about calcium nitrate and urea is made: the compatibility is limited, moisture pick up must be absolutely avoided, therefore quote: “consider the relative humidity during blending”.
It is important to notice that blending UAS with calcium nitrate is much more an issue, due to the formation of UAN (urea ammonium nitrate) and subsequent liquid phase formation. Indeed, ammonium sulphate from UAS can react with calcium nitrate to form ammonium nitrate and calcium sulphate, and ammonium nitrate forms the very hygroscopic UAN double salt with urea as described here above.(NH4)2SO4+Ca(NO3)2=>CaSO4+2NH4NO3 
In the past, there has been development of a technique allowing blending urea with e.g. TSP. This was based on a sulphur coating of one or both components. Typically urea is coated with about 20% of molten sulphur, in order to create a strong barrier isolating the urea from the superphosphate.
The opposite approach was possible, i.e. to produce sulphur-coated TSP in order to make it blendable with urea. Such an approach suffers significant drawbacks, that the present invention overcomes. First of all such a layer of sulphur, if well tight, creates a delayed release effect since sulphur is insoluble in water. It means that one compound, either urea or TSP depending on which one has been sulphur-coated, will have some delayed release effect which is not necessarily the aim. Moreover, the sulphur shell will remain for very long in the field, having virtually no positive effect for the fertilization. In order to get a proper sulphur-coating, typically 20% of sulphur needs to be applied. If less, the coating is not thick enough and will be imperfect, leading to degradation over time of the blend. This coating act then as a diluent of the fertilizer without bringing extra fertilization value. Moreover, sulphur is incompatible with ammonium nitrate, therefore such sulphur coating technique is anyway not applicable to ammonium nitrate containing blends.
Based on the same principle of an insoluble coating, one can mention the possibilities of blending some polymer-coated urea with e.g. ammonium nitrate. Such polymer-coated urea product is available for example in the North American market, see for example coatings as described in International patent application WO2012/064730. It is produced to get a slow release effect of nitrogen urea. Thanks to this slow release coating, it can be blended with most other products, but by nature its nitrogen will be released with delay compared to the other nutrients. Moreover, such a polymer coating has in itself no fertilizing value and dilutes by several percent the nitrogen content of urea.